1. Field of the Invention
The present invention relates to methods for treating wastewater and apparatuses therefor, and more particularly, to methods for removing phosphorus and nitrogen from wastewater and apparatuses therefor.
2. Description of the Related Art
The activated sludge process is a typical treatment process for removing organic substances from wastewater, and an anaerobic-aerobic activated sludge process is used as a conventional method for simultaneously removing phosphorus anions and organic substances from wastewater.
An example of a wastewater treatment apparatus by the anaerobic-aerobic activated sludge process is shown in FIG. 6. This wastewater treatment apparatus includes a primary settling tank 2, an anaerobic tank 3 in which activated sludge releases phosphoric acid ion from cells into wastewater by a biological phosphorus-releasing reaction, an aerobic tank 4 in which activated sludge takes up phosphoric acid ion into cells by a biological phosphorus-uptake reaction, and a final settling tank 6. The amount of phosphorus taken up at the aerobic stage is greater than the amount of phosphorus released at the anaerobic stage, and a difference between these amounts corresponds to the amount of phosphorus removed.
In the primary settling tank 2, relatively large and heavy solids contained in wastewater 1 are removed. After being subjected to the biological phosphorus-releasing reaction at the anaerobic stage and the biological phosphorus-uptake reaction at the aerobic stage, phosphorus in the wastewater is transformed into a constituent of the sludge, and is finally discharged as excess sludge from the wastewater treatment apparatus. Meanwhile, organic substances in the wastewater are removed both at the anaerobic stage and at the aerobic stage.
The phosphorus removal from wastewater using a wastewater treatment apparatus by means of the anaerobic-aerobic activated sludge process has the following drawbacks. That is, when the organic substance concentration in the wastewater is decreased, for example, by the influx of rainwater, since the wastewater is diluted while being supplied with oxygen, the organic substances in the wastewater, which have been oxidized and diluted, are fed to the anaerobic stage. As a result, since the organic substance concentration is decreased in comparison with the phosphoric acid ion concentration, the rate of the phosphorus-releasing reaction at the anaerobic stage and the rate of the phosphorus-uptake reaction at the aerobic stage are decreased, resulting in deterioration in the quality of treated water.
Since solid waste components of the wastewater are subjected to sedimentation in the primary settling tank, the wastewater supplied to the anaerobic tank from the primary settling tank containing mainly soluble pollutants. Therefore, when the ratio of the organic substance concentration to the phosphoric acid ion concentration is low with respect to the soluble pollutants in the wastewater to be fed to the anaerobic stage, the rate of the phosphorus-releasing reaction at the anaerobic stage and the rate of the phosphorus-uptake reaction at the aerobic stage are also decreased, resulting in deterioration in the quality of treated water.
In order to cope with such problems, a method may be employed in which an organic chemical agent such as methanol, together with the wastewater, is supplied to the anaerobic stage, and thus, by compensating for the shortage of the organic substance concentration in the wastewater, decreases in the rate of the phosphorus-releasing reaction at the anaerobic stage and the rate of the phosphorus-uptake reaction at the aerobic stage are prevented.
In the conventional method of adding an organic chemical agent to the anaerobic stage, although in general, relatively inexpensive methanol is used, the cost for the chemical agent still increases operating expenses. Moreover, since methanol is a Class IV hazardous substance, safety must be taken into consideration in handling and storage facilities, and facilities for receipt and supply must be prepared, resulting in an increased cost of equipment and difficulty in handling.
With respect to both the phosphorus-releasing reaction at the anaerobic stage and the phosphorus-uptake reaction at the aerobic stage, substantially large amounts of organic substances are required. In the phosphorus removal treatment by the anaerobic-aerobic activated sludge process, if there is a shortage of organic substances, satisfactory phosphorus removal cannot be achieved. Therefore, it is effective for improving the results of phosphorus removal treatment to introduce the organic substances contained in the wastewater fed into the primary settling tank and removed in the primary settling tank, that is, organic substances mainly composed of solids, to the anaerobic stage or both the anaerobic stage and the aerobic stage.
As a conventional method of operating facilities having such an effect, there is a method in which by decreasing the number of primary settling tanks used in wastewater treatment facilities having a plurality of treatment lines, operation is performed at increased water surface loading with respect to the primary settling tanks so that suspended solid (SS) concentration in the primary settling tank effluent is increased, and thus organic substances flowing into the anaerobic stage are increased.
However, it is difficult to accurately control the amount of solid organic substances that flow into the anaerobic stage by changing the number of primary settling tanks in operation.
In order to solve the problems described above, the inventor has already applied for patents which disclose techniques for satisfactory phosphorus removal treatment by increasing an amount of organic substances to be supplied to anaerobic tanks. One such method for removing phosphorus from wastewater, in which at least a portion of sediment in a primary settling tank is fed to an anaerobic tank, and a method for removing phosphorus from wastewater, in which the sediment is subjected to ultrasonic treatment before being fed to the anaerobic tank, are disclosed in Japanese Patent Application No. 9-133989. Another such method for removing phosphorus from wastewater, in which the sediment is subjected to ozone treatment before being fed to the anaerobic tank, is disclosed in Japanese Patent Application No. 9-133988. However, when the sediment in the primary settling tank is introduced to the anaerobic tank as it is, since particles of solids contained in the sedimented sludge are relatively large and have a small surface area per unit weight, the rate of decomposition/treatment in a biological reactor such as an anaerobic tank is relatively low. When the sediment in the primary settling tank is fed to the anaerobic tank after ultrasonic treatment, solids, as the sediment, contain microorganisms enclosed by cell walls and/or cell membranes, and cellulose, which are relatively rigid and are not easily broken by vibration treatment. Therefore, the particle size of the solids introduced to the anaerobic tank is not sufficiently reduced. Furthermore, when the sediment in the primary settling tank is fed to the anaerobic tank after ozone treatment, a portion of organic substances contained in the sediment is oxidized and mineralized, and thus loses effectiveness as organic substances. Since a relatively large amount of electric power, approximately 10 to 15 kWh/kg O3, is also consumed for the generation of ozone, operating expenses increase.
As a conventional method for simultaneously removing nitrogen compounds and organic substances, a biological nitrification-denitrification process is used.
An example of a wastewater treatment apparatus by the biological nitrification-denitrification process is shown in FIG. 14. This wastewater treatment apparatus includes a primary settling tank 2, a nitrification tank 4 for oxidizing (nitrifying) nitrogen compounds contained in wastewater to nitrate nitrogen or to nitrite nitrogen, a denitrification tank 3 for reducing (denitrifying) nitrate nitrogen or nitrite nitrogen to nitrogen gas, and a final settling tank 7. After relatively large and heavy solids are removed from wastewater 1 in the primary settling tank 2, nitrification and denitrification are performed, and nitrogen is released into the atmosphere as nitrogen gas, and thus nitrogen is removed from wastewater. Meanwhile, organic substances in the wastewater are removed both at the denitrification stage and at the nitrification stage.
The nitrogen removal from wastewater using a wastewater treatment apparatus by the biological nitrification-denitrification process has the following drawbacks. That is, when the organic substance concentration in the wastewater is decreased, for example, by the influx of rainwater, since the wastewater is diluted while being supplied with oxygen, the organic substances in the wastewater, which have been oxidized and diluted, are fed to the denitrification stage. As a result, since the organic substance concentration is further decreased in comparison with the nitrogen concentration, the rate of denitrification reaction at the denitrification stage is decreased, resulting in deterioration in the quality of treated water.
Since solid particles are subjected to sedimentation in the primary settling tank, the wastewater fed to the denitrification stage through the primary settling tank is mainly composed of soluble pollutants. Therefore, when the ratio of the organic substance concentration to the nitrogen concentration is low with respect to the soluble wastes in the wastewater to be fed to the denitrification stage, the rate of nitrogen removal reaction at the denitrification stage is also decreased, resulting in deterioration in the quality of treated water.
In order to cope with such problems, a method is used, in which an organic chemical agent such as methanol, together with the wastewater, is fed to the denitrification stage, and thus, by compensating for the shortage of the organic substance concentration in the wastewater, a decrease in the rate of denitrification reaction is prevented.
In the conventional method of adding an organic chemical agent to the denitrification stage, although in general methanol, which is relatively inexpensive, is used, the cost for the chemical agent still increases operating expenses. Moreover, since methanol is a Class IV hazardous substance, safety must be taken into consideration in handling, and storage facilities and facilities for receipt and supply must be prepared, resulting in increased cost of equipment and difficulty in handling.
When the ratio of the organic substance concentration to the nitrogen concentration is low with respect to the wastewater supplied to the denitrification stage, it is difficult to cause satisfactory nitrogen removal reactions, in particular, denitrification reactions, because of a shortage of organic substances, and thus satisfactory nitrogen removal cannot be achieved. Therefore, it is effective in improving the results of nitrogen removal treatment to introduce the organic substances contained in the wastewater fed into the primary settling tank and removed in the primary settling tank, that is, organic substances mainly composed of solids, to the denitrification stage.
As a conventional method for operating facilities for producing such effects, there is a method in which, by decreasing the number of primary settling tanks used in wastewater treatment facilities having a plurality of treatment lines, operation is performed at increased water surface loading with respect to the primary settling tanks so that suspended solids (SS) concentration in the primary settling tank effluent is increased, and thus the amount of organic substances flowing into the denitrification stage is increased.
However, it is difficult to accurately control the amount of solid organic substances that flow into the denitrification stage by changing the number of primary settling tanks in operation.
In order to solve the problems described above, the inventor has already applied for patents which disclose techniques for satisfactory nitrogen removal treatment by increasing the amounts of organic substances to be supplied to denitrification tanks or anaerobic tanks. One such method for removing nitrogen from wastewater, in which at least a portion of sediment in a primary settling tank is fed to a denitrification tank or an anaerobic tank, and a method for removing nitrogen from wastewater, in which the sediment is subjected to ultrasonic treatment before being fed to the denitrification tank or the anaerobic tank, are disclosed in Japanese Patent Application No. 9-133989. Another such method for removing nitrogen from wastewater, in which the sediment is subjected to ozone treatment before being fed to the denitrification tank or the anaerobic tank, is disclosed in Japanese Patent Application No. 9-133988. However, when the sediment in the primary settling tank is introduced to the denitrification tank or the anaerobic tank as it is, since particles of solids contained in the sedimented sludge are relatively large and have a small surface area per unit weight, the rate of decomposition/treatment in a biological reactor, such as a denitrification tank or an anaerobic tank, is relatively low. When the sediment in the primary settling tank is fed to the denitrification tank or the anaerobic tank after ultrasonic treatment, solids, as the sediment, contain microorganisms enclosed by cell walls and/or cell membranes, and cellulose, which are relatively rigid and are not easily broken by vibration treatment. Therefore, the particle size of the solids introduced to the denitrification tank or the anaerobic tank is not sufficiently reduced. Furthermore, when the sediment in the primary settling tank is fed to the denitrification tank or the anaerobic tank after ozone treatment, a portion of organic substances contained in the sediment is oxidized and mineralized, and thus loses effectiveness as organic substances. Since a relatively large amount of electric power, approximately 10 to 15 kWh/kg O3, is also consumed for the generation of ozone, operating expenses increase.
It is an object of the present invention to provide methods for treating wastewater and apparatuses therefor, in which effective wastewater treatment can be performed in an economical manner.
According to a first aspect of the present invention, a method for treating wastewater includes the steps of: removing phosphorus from wastewater, using a wastewater treatment apparatus including a settling tank, an anaerobic tank, and an aerobic tank; crushing at least a portion of sediment in the settling tank; and supplying the crushed sediment to the anaerobic tank.
According to a second aspect of the present invention, a wastewater treatment apparatus includes: a settling tank for removing solids contained in wastewater; an anaerobic tank, in which effluent from the settling tank is introduced and activated sludge releases phosphoric acid ion from cells into the wastewater by a biological phosphorus-releasing reaction; an aerobic tank in which effluent from the anaerobic tank is introduced and activated sludge takes up phosphoric acid ion into cells by a biological phosphorus-uptake reaction; a line for withdrawing sediment from the settling tank; a crushing device for crushing the withdrawn sediment; and a line for supplying the crushed sediment to the anaerobic tank.
According to a third aspect of the present invention, a method for treating wastewater includes the steps of: removing phosphorus from wastewater, using a wastewater treatment apparatus including a primary settling tank, an anaerobic tank, an aerobic tank, and a final settling tank; returning effluent from the aerobic tank to the primary settling tank; returning sludge withdrawn from the final settling tank to the primary settling tank; and sending sludge withdrawn from the primary settling tank to the anaerobic tank or the aerobic tank.
According to a fourth aspect of the present invention, a method for treating wastewater includes the steps of: removing phosphorus from wastewater, using a wastewater treatment apparatus including a primary settling tank, an anaerobic tank, an aerobic tank, and a final settling tank; returning effluent from the aerobic tank to the primary settling tank; and sending sludge withdrawn from the primary settling tank to the anaerobic tank or the aerobic tank.
According to a fifth aspect of the present invention, a method for treating wastewater includes the steps of: removing phosphorus from wastewater, using a wastewater treatment apparatus including a primary settling tank, an anaerobic tank, an aerobic tank, and a final settling tank; returning sludge withdrawn from the final settling tank to the primary settling tank; and sending sludge withdrawn from the primary settling tank to the anaerobic tank or the aerobic tank.
According to a sixth aspect of the present invention, a wastewater treatment apparatus includes: a primary settling tank for removing solids contained in wastewater; an anaerobic tank, in which effluent from the settling tank is introduced and activated sludge releases phosphoric acid ion from cells into the wastewater by a biological phosphorus-releasing reaction; an aerobic tank in which effluent from the anaerobic tank is introduced and activated sludge takes up phosphoric acid ion into cells by a biological phosphorus-uptake reaction; a final settling tank for settling wastewater treated in the aerobic tank; a line for returning effluent from the aerobic tank to the primary settling tank; a line for returning sludge withdrawn from the final settling tank to the primary settling tank; and a line for sending sludge withdrawn from the primary settling tank to the anaerobic tank or the aerobic tank.
According to a seventh aspect of the present invention, a wastewater treatment apparatus includes: a primary settling tank for removing solids contained in wastewater; an anaerobic tank, in which effluent from the settling tank is introduced and activated sludge releases phosphoric acid ion from cells into the wastewater by the biological phosphorus-releasing reaction; an aerobic tank in which effluent from the anaerobic tank is introduced and activated sludge takes phosphoric acid ion into cells by the biological phosphorus-uptake reaction; a final settling tank for settling wastewater treated in the aerobic tank; a line for returning effluent from the aerobic tank to the primary settling tank; and a line for sending sludge withdrawn from the primary settling tank to the anaerobic tank or the aerobic tank.
According to an eighth aspect of the present invention, a wastewater treatment apparatus includes: a primary settling tank for removing solids contained in wastewater; an anaerobic tank, in which effluent from the settling tank is introduced and activated sludge releases phosphoric acid ion from cells into the wastewater by a biological phosphorus-releasing reaction; an aerobic tank in which effluent from the anaerobic tank is introduced and activated sludge takes up phosphoric acid ion into cells by a biological phosphorus-uptake reaction; a final settling tank for settling wastewater treated in the aerobic tank; a line for returning sludge withdrawn from the final settling tank to the primary settling tank; and a line for sending sludge withdrawn from the primary settling tank to the anaerobic tank or the aerobic tank.
According to a ninth aspect of the present invention, a method for treating wastewater includes the steps of: removing nitrogen from wastewater, using a wastewater treatment apparatus including a settling tank and a denitrification tank; crushing at least a portion of sediment in the settling tank; and supplying the crushed sediment to the denitrification tank.
According to a tenth aspect of the present invention, a method for treating wastewater includes the steps of: removing nitrogen from wastewater, using a wastewater treatment apparatus including a settling tank and an anaerobic tank, and a nitrification tank; crushing at least a portion of sediment in the settling tank; and supplying the crushed sediment to the anaerobic tank.
According to an eleventh aspect of the present invention, a wastewater treatment apparatus includes: a settling tank for removing solids contained in wastewater; a denitrification tank, in which effluent from the settling tank is introduced and nitrate nitrogen or nitrite nitrogen in wastewater is reduced to nitrogen gas; a line for withdrawing sediment in the settling tank; a crushing device for crushing the withdrawn sediment; and a line for supplying the crushed sediment to the denitrification tank.
According to a twelfth aspect of the present invention, a wastewater treatment apparatus includes: a settling tank for removing solids contained in wastewater; an anaerobic tank, in which effluent from the settling tank is introduced, nitrate nitrogen or nitrite nitrogen in the wastewater is reduced to nitrogen gas, and activated sludge releases phosphoric acid ion from cells into the wastewater by a biological phosphorus-releasing reaction; a line for withdrawing sediment from the settling tank; a crushing device for crushing the withdrawn sediment; and a line for supplying the crushed sediment to a denitrification tank.
According to a thirteenth aspect of the present invention, a wastewater treatment apparatus includes: a primary settling tank for removing solids contained in wastewater; a denitrification tank, in which effluent from the primary settling tank is introduced, and nitrate nitrogen or nitrite nitrogen in the wastewater is reduced to nitrogen gas; a nitrification tank, in which effluent from the denitrification tank is introduced, and nitrogen compounds in the wastewater are oxidized to nitrate nitrogen or nitrite nitrogen; a final settling tank for settling effluent from the nitrification tank; and a line for returning a portion of effluent from the nitrification tank or a portion of effluent from the final settling tank to the primary settling tank.